i've experimented in da hills w/ the only bivi sack or only lightweight sleepign bag and from now on will always go w/ the sleeping bag...the sack always ends up being colder, but then i've never unexpectedly been hit by a rainstorm.  
  
No sleeping bag?   
  
Fawk that shite. 
  
The extra clothes you'll to keep your teeth from shattering will undoubtedly outweigh a nice, warm, summer bag. 
  
Skimp on the clothes, not the bag. No sleep = shitty performance and a shitty time.         
Here here!  
  
What about those half-bags?  I've heard some folks swear by them.